Spinal deformities, which include rotation, angulation, and/or curvature of the spine, can result from various disorders, including, for example, scoliosis (abnormal curvature in the coronal plane of the spine), kyphosis (backward curvature of the spine), and spondylolisthesis (forward displacement of a lumbar vertebra). Other causes of an abnormally shaped spine include trauma and spinal degeneration with advancing age. Early techniques for correcting such deformities utilized external devices that applied force to the spine in an attempt to reposition the vertebrae. These devices, however, resulted in severe restriction and in some cases immobility of the patient. Furthermore, current external braces have limited ability to correct the deformed spine and typically only prevent progression of the deformity. Thus, to avoid this need, doctors developed several internal fixation techniques to span across multiple vertebrae and force the spine into a desired orientation. Additional fixation is beneficial in cases in which the bone quality is poor, such as patients with osteoporotic bone. The poor bone quality reduces the strength of the bone to implant interface.
To fix the spine, surgeons attach one or more fixation elements (typically rods or plates) to the spine at several fixation sites to correct and stabilize the spinal deformity, prevent reoccurrence of the spinal deformity, and stabilize weakness in trunks that results from degenerative discs and joint disease, deficient posterior elements, spinal fracture, and other debilitating problems. Bone screws are typically used to anchor the spinal rods or plates at the various fixation sites. Once anchored, the rod-based systems are under stress and subjected to significant forces, known as cantilever pullout forces. As a result, surgeons are always concerned about the possibility of the implant loosening or the bone screws pulling out of the bone. Thus, surgeons generally seek to attach implants in the most secure and stable fashion possible while at the same time addressing a patient's specific anatomy. In some instances, motion of the spine causes stress at the interface between the bone screws and the bone, resulting in loosening of the bone screws.
Accordingly, there is a need in this art for improved methods and devices for bone screw fixation.